Thermoplastic substrates such as polycarbonates and poly(methyl methacrylate) are generally characterized by their many advantageous properties such as clarity, high ductility, high heat deflection temperature as well as dimensional stability. Many of these materials are transparent and are conventionally employed as replacements for glass in many commercial applications. However, such materials easily scratch and abrade, resulting in possible decrease in transparency. They are also susceptible to degradation by ultraviolet (UV) light, giving rise to problems like yellowing and whitening of the substrate surface. One solution to such problems is by applying a mar resistant coating composition containing a UV absorber onto the surface of resin substrates.
However, the mar resistant coating films containing a UV absorber have the drawbacks that the UV absorber can bleed out or flow away, failing to exert its desired properties, and the addition of the UV absorber detracts from essential mar resistance performance. In one approach to address these drawbacks, a UV absorber is attached and anchored to a binder component in the mar resistant coating film to prevent the film from losing weather resistance and mar resistance due to bleed-out and outflow. To this end, the UV absorber must be designed in compliance with the main chain structure and crosslinking mode of the binder.
For example, silicone hardcoat compositions cure into hardcoat films with good mar resistance and durability via crosslinking reaction involving silanol (SiOH) condensation of a precursor in the form of a hydrolytic condensate of an alkoxysilane. Of the UV absorbers used in such silicone hardcoat compositions, UV absorbers having introduced reactive groups for silanol crosslinking are known from Patent Documents 1 to 4. However, the process for preparing the reactive silyl-modified UV absorber has some problems including a multi-stage synthetic route and removal of a hydrosilylation catalyst. Generally, no tight adhesion to a resin substrate is achievable unless a silicone hardcoat film is laid on the substrate via a primer layer. This necessitates an extra process including preparation, coating and curing of the primer.
Another known mar resistant coating is a photo-curable (meth)acrylic coating composition. The photo-curable (meth)acrylic coating composition comprises at least one polyfunctional (meth)acrylate and a photopolymerization initiator. Crosslinking takes place via photopolymerization of (meth)acrylic groups on the polyfunctional (meth)acrylate, forming a film. Reactive group-introduced UV absorbers are added to such photo-curable (meth)acrylic coating compositions as reported in Patent Documents 5 to 9. The photo-curable (meth)acrylic coating composition comprising the reactive group-introduced UV absorber can be directly applied and cured to a resin substrate without a need for primers which are required in the above-mentioned silicone hardcoat compositions. However, there still remain some outstanding issues.
Patent Documents 5 and 6 disclose alkoxysilyl-bearing dibenzoyl resorcinol derivatives as another example of the reactive group-introduced UV absorber. Since the alkoxysilyl group in this UV absorber is not involved in crosslinking reaction of the photo-curable (meth)acrylic coating, the cured film is estimated short in mar resistance. In fact, these documents do not refer to mar resistance. With respect to long-term weather resistance, there is a possibility of the UV absorber bleeding out because the UV absorber has not been reacted with the binder.
Patent Documents 7 and 8 describe silsesquioxanes having a (meth)acrylic functional benzotriazole as a UV absorbing group. Although the solubility of the UV absorber in a polyfunctional (meth)acrylate is improved over the unmodified benzotriazole base UV absorbers, it is still insufficient for high loadings. This UV absorber has a relatively large moiety such as silsesquioxane skeleton which does not contribute to UV absorption. That is, this UV absorber has a low UV absorbing group content, provided that the “UV absorbing group content” is defined as a proportion of UV absorbing groups per molecular weight of the absorber. Inevitably the UV absorber must be added in a large amount in order to obtain a necessary UV absorbing capacity, which tends to detract from the desired properties (e.g., mar resistance and adhesion) except weather resistance.
Patent Document 9 discloses a photo-curable (meth)acrylic polymer having both a UV absorbing group and a (meth)acryloyl group as side chains. The (meth)acrylic polymer reacts with a polyfunctional (meth)acrylate in the photo-curable (meth)acrylic coating composition whereby it is anchored in the film. The UV absorber also has improved solubility in the polyfunctional (meth)acrylate, but a low UV absorbing group content because of the polymer form. The UV absorber must be added in a large amount, which tends to detract from mar resistance and adhesion.
Also known are (meth)acrylic functional UV absorbers having a high UV absorbing group content and a relatively low molecular weight. Typical of commercially available absorbers is 2-[2′-hydroxy-5′-(methacryloyloxyethyl)phenyl]-2H-benzotriazole (trade name RUVA-93 from Otsuka Chemical Co., Ltd.).
Patent Documents 10 and 11 disclose benzotriazole and benzophenone base UV absorbers in which an unsaturated double bond is attached to a urethane bond via an alkylene chain, respectively. It is generally known that benzophenone and resorcinol base UV absorbers exhibit insufficient UV absorbing capacity while some benzotriazole base UV absorbers are toxic. Those benzotriazole base UV absorbers which are safe have a low solubility, with their amount of addition being limited.
In connection with a photo-curable (meth)acrylic coating composition which can be applied directly on a resin substrate without a need for primers, it is desired to have a UV absorber which is fully soluble in a polyfunctional (meth)acrylate, has a high UV absorbing group content, and does not compromise the mar resistance and adhesion of a coating film.